Unit

A type used when a return value is of no interest e.g. the return type of `printf` is `unit`.

Similar to the void type is C# and C++.

A do binding or the return value of a top level module must be unit.

1: 
2: 
let function1 x y = x + y
function1 10 20 |> ignore

Because function1 returns a value other than unit it must be ignored.